1. Technical Field
The present invention generally relates to lubricating oil compositions.
2. Description of the Related Art
Automobile spark ignition and diesel engines have valve train systems, including valves, cams and rocker arms, which present special lubrication concerns. It is extremely important that the lubricant, i.e., the engine oil, protects these parts from wear. It is also important for the engine oils to suppress the production of deposits in the engines. Such deposits are produced from non-combustibles and incomplete combustion of hydrocarbon fuels (e.g., gasoline and diesel fuel oil) and by the deterioration of the engine oil employed.
Engine oils typically use a mineral oil or a synthetic oil as a base oil. However, simple base oils alone do not provide the necessary properties to provide the necessary wear protection, deposit control, etc., required to protect internal combustion engines. Thus, base oils are formulated with various additives, for imparting auxiliary functions, such as ashless dispersants, metallic detergents (i.e., metal-containing detergents), antiwear agents, antioxidants (i.e., oxidation inhibitors), viscosity index improvers and the like to give a formulated oil (i.e., a lubricating oil composition).
A number of such engine oil additives are known and employed in practice. For example, zinc dialkyldithiophosphates are usually contained in the commercially available internal composition engine oils, especially those used for automobiles, because of their favorable characteristics as an antiwear agent and performance as an oxidation inhibitor.
However, a problem associated with the use of zinc dialkyldithiophosphate is that their phosphorus and sulfur derivatives poison the catalyst components of the catalytic converters. This is a major concern as effective catalytic converters are needed to reduce pollution and to meet governmental regulation designed to reduce toxic gases such as, for example, hydrocarbons, carbon monoxide and nitrogen oxides, in internal combustion engine exhaust emissions. Such catalytic converters generally use a combination of catalytic metals, e.g., platinum and metal oxides, and are installed in the exhaust streams, e.g., the exhaust pipes of automobiles, to convert the toxic gases to nontoxic gases. As previously mentioned, these catalyst components are poisoned by the phosphorus and sulfur components, or the phosphorus and sulfur decomposition product of the zinc dialkyldithiophosphate; and accordingly, the use of engine oils containing phosphorus and sulfur additives may substantially reduce the life and effectiveness of catalytic converters.
There is also governmental and automotive industry pressure towards reducing the phosphorus and sulfur content. For example, current GF-4 motor oil specifications require a finished oil to contain less than 0.08 wt % and 0.7 wt % phosphorus and sulfur, respectively, and CJ-4 motor oil specifications, the most current generation heavy duty diesel engine oil, require an oil to contain less than 0.12 wt % and 0.4 wt % phosphorus and sulfur, respectively, and 1.0 wt % sulfated ash. It is widely believed that lowering these limits may have a serious impact on engine performance, engine wear, and oxidation of engine oils. This is because historically a major contributor to the phosphorus content in engine oils has been zinc dialkyldithiophosphates. Accordingly, it would be desirable to eliminate the amount of zinc dialkyldithiophosphate in lubricating oils, thus reducing catalyst deactivation and hence increasing the life and effectiveness of catalytic converters while also meeting future industry standard proposed phosphorus and sulfur contents in the engine oil. However, simply decreasing the amount of zinc dialkyldithiophosphate presents problems because this necessarily lowers the antiwear properties and oxidation inhibition properties of the lubricating oil. Therefore, it is necessary to find a way to reduce or eliminate phosphorus and sulfur content while still retaining the antiwear properties of the higher phosphorus and sulfur content engine oils.
U.S. Patent Application Publication No. 20070111908 (“the '908 application”) discloses a lubricating oil composition containing an oil of lubricating viscosity, at least one succinimide dispersant derived from a polyalkylene compound having from about 50 to about 85% vinylidene double bonds in the compound, a metal containing detergent, at least one wear reducing agent, at least one antioxidant, and a hydrocarbon soluble titanium compound which is a reaction product of a titanium alkoxide and an about C6 to about C25 carboxylic acid as a friction modifier, wherein the lubricating oil composition is substantially free of molybdenum compounds. The '908 application further discloses that the wear reducing agent is at least one metal dihydrocarbyl dithiophosphate compound such as a zinc dihydrocarbyl dithiophosphate.
U.S. Patent Application Publication No. 20070149418 (“the '418 application”) discloses a lubricating oil composition containing (a) an oil of lubricating viscosity, (b) a friction modifier selected from the group consisting essentially of an organomolybdenum friction modifier, a glycerol ester friction modifier, and mixtures thereof, and (c) an antiwear agent comprising an amount of at least one hydrocarbon soluble titanium compound effective to provide an increase in antiwear properties of the lubricant composition greater than an increase in antiwear properties of the lubricant composition devoid of the hydrocarbon soluble titanium compound, wherein the compound is essentially devoid of sulfur and phosphorus atoms. The '418 application further discloses that the hydrocarbon soluble titanium compound is a reaction product of a titanium alkoxide and an about C6 to about C25 carboxylic acid. All of the examples disclosed in the '418 application disclose a hydrocarbon soluble titanium compound in combination with a zinc dithiophosphate.
Therefore, as demand for further decrease of the phosphorus content and a limit on the sulfur content of lubricating oils is very high, this reduction cannot be satisfied by the present measures in practice and still meet the severe antiwear and oxidation-corrosion inhibiting properties required of today's engine oils. Accordingly, it would be desirable to develop lubricating oil compositions having relatively low levels or free of any phosphorus content while also having relatively low levels of sulfur and sulfated ash but which still provide the needed wear protection now provided by lubricating oils containing a zinc dialkyldithiophosphate.